The present invention is directed to cellular radio communications systems, and more particularly to systems which employ code division multiple access (CDMA) to distinguish different communications within a cell from one another.
In a cellular radio communications system, a geographical area is divided into cells where neighboring cells are generally allocated different components of a communications parameter, to avoid interference. In early cellular systems, this communication parameter was the carrier frequency over which the communications were transmitted. Thus, to avoid interference, neighboring cells used different sets of mutually exclusive communications frequencies. Cells that were at a distance of two or more cells away from a given cell could reuse the same frequencies as the given cell. The further apart the cells reusing the same frequency were located, the lower the interference level between them.
The total number of different frequencies that are required to construct a cell pattern having an acceptable level of interference reduces the number of frequencies that are available for use within the individual cells. For example, one common cell arrangement employs a 21-cell pattern to avoid interference. In this type of arrangement, the cells are grouped in clusters of 21 contiguous cells. Each cell within a cluster must use a different set of frequencies from the other 20 cells in that cluster. If 420 frequencies are available for use within such a communications system, the number of frequencies that can be used in each cell is 420/21=20.
With current state-of-the-art techniques, digitized voice transmission is preferred to the transmission of analog waveforms, since it is more tolerant of interference and thus permits a tighter frequency re-use pattern with consequent increase in capacity. Furthermore, with the transmission of information in digital form, each transmission frequency can be divided into time slots, with each time slot carrying a different communication. Thus, if each frequency is divided into three or four repeating timeslots, the number of communications that can be transmitted at any given time is effectively tripled or quadrupled. This approach to increasing system capacity is known as time division multiple access (TDMA).
Furthermore, when digital transmission is used, error correction coding can be employed to increase the interference tolerance. If a systematic code is employed for error correction, a number of parity bits are transmitted in addition to data bits which represent the digitized speech. More preferably, however, a non-systematic code is employed, in which all of the bits of digitized speech are converted into code words. For example, if a 128,7 block encoding technique is employed, each group of seven data bits is converted into a 128-bit code word and transmitted as such. At the receiver, each received 128-bit codeword is reconverted into the original 7-bit piece of data, e.g. speech information. Even if some of the bits of the received 128-bit codeword are erroneous, due to interference, the original data can still be easily recovered.
Unfortunately, the use of error correction coding increases the number of bits that are transmitted for each piece of information. As a result, the transmission bandwidth is widened, thereby reducing the number of frequency channels that are available without overlap. Thus, there is a tradeoff between increased interference tolerance, which permits more frequent reuse of frequencies, and a reduction in the number of frequency channels available.
At one extreme of this approach, the amount of coding that is employed is so effective that interference levels which are equal to or greater than the desired signal can be tolerated, and signal overlap can therefore be permitted. A system which operates with this approach is known as code division multiple access (CDMA). An exemplary CDMA communications system is disclosed in U.S. patent application Ser. No. 07/628,359, filed Dec. 17, 1990, and U.S. patent application Ser. No. 07/739,446, filed Aug. 2, 1991, which is a continuation-in-part application thereof. In the system disclosed in those applications, the ability to tolerate an increased number of interfering signals, to thereby achieve an increase in system capacity, is provided through the use of a subtractive demodulation process. Generally speaking, a receiver in this type of system does not operate to decode only a single desired signal in the presence of a large number of interfering signals. Rather, a number of received signals, both interfering and desired, are successively decoded in order of received signal strength. After being decoded, each interfering signal is recoded and subtracted from the received signal, to thereby reduce the interference that is present when the desired signal is decoded.
With this approach, a larger number of signals, which are differently enciphered to provide a means of discriminating them from one another, are permitted to overlap. The capacity of such a system is not limited by theoretical bounds, but rather by the amount of signal processing resources that are available to demodulate a multiplicity of signals. Accordingly, it is desirable to provide a radio communications system which affords increased system capacity in terms of the number of simultaneous communications that can be reliably transmitted and received, while at the same time minimizing receiver activity to reduce the processing requirements of the receiver.
One approach that has been used in the past to reduce the presence of interfering signals is to turn off the transmitters associated with a momentarily silent party in a two-party conversation. With this approach, the number of conversations can then be doubled before the interference reaches the original level. This technique is known as discontinuous transmission. In the past, it has been employed in non-CDMA systems, such as the time division multiple access pan-European digital cellular system, which is known as the GSM system.
A difficulty is presented when discontinuous transmission is employed in CDMA systems, however, due to the high time synchronization accuracy which must be maintained to successfully decode CDMA signals. If the transmission of a signal is interrupted for more than a short period of time, the timing of a receiver may drift to the extent that it does not immediately recognize the point at which transmission resumes. In the GSM system, the use of discontinuous transmission is facilitated by the transmission of a special code at the beginning of a transmitter shut-down period, to notify the receiver of the interruption in transmission. However, this indication is not employed to reduce the activity of the receiver. Rather, it is used to mute the audio output from the receiver during the period of no input signal from the transmitter.
U.S. Pat. No. 4,901,307 discloses a CDMA system in which the transmitter is controlled to transmit in an intermittent fashion with a variable duty factor that is contingent upon the speech activity level. In the operation of this system, the transmitter is not switched off completely during speech activity. Rather, the duty factor is established such that the transmissions take place even during periods of speech inactivity to provide enough information to maintain receiver synchronization. The purpose of this approach is to reduce the total interference level while maintaining receiver synchronization, rather than to reduce receiver signal processing resources.